The present invention claims priority from Japanese Patent Application No.9-238338 filed Sep. 3, 1997, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a self-healing for improving the reliability of ABR (Available Bit Rate) communication services in an ATM (Asynchronous Transfer Mode) transmission.
2. Description of Related Art
As a technique for improving the reliability of communication services in an ATM transmission, the self healing process for automatically setting a new communication route by preparing a bypass of an original communication route in which a failure occurs has been known. A conventional ATM cell switch device which can perform such processing is shown in FIG. 1.
FIG. 1 is a block diagram showing a construction of the conventional ATM cell switch device 11 together with two ATM terminals, in which one of the ATM terminals transmits data cell as a source terminal 21 and the other ATM terminal receives the data cell as a destination terminal 22. Only portions of the ATM cell switch device 11 which are related to the data cell transfer from the source terminal 21 to the destination terminal 22 are shown in FIG. 1.
The ATM cell switch device 11 includes a receiver circuit 13a which receives a cell including data cell and RM (Resource Management) cell for network control in relation to the data cell, a source terminal transmission rate measuring circuit 14 and a switch (SW) circuit 15 which receives the cell transmitted from the receiving circuit 13a through the source terminal transmission rate measuring circuit 14. In the SW circuit 15, a routing is performed on the basis of a VPI and VCI of a cell header of the cell and the cell is transmitted to the destination terminal 22 through a transmitter circuit 16a which is also included in the ATM cell switch device 11. The RM cell aliased by the destination terminal 22 is transmitted to the source terminal 21 through a receiving circuit 16b, the SW circuit 15 and a transmitter circuit 13b. 
The source terminal transmission rate measuring circuit 14 counts the ATM cells on the receiving side and outputs the count to a control circuit 12 as a measured source terminal transmission rate signal. The measured source terminal transmission rate signal is used during a time period in which self healing is being executed as a source terminal transmission rate of a path to be monitored when the path to be monitored is a failure path and as an amount of use of frequency band of a bypass route when the path to be monitored is on a bypass route.
The control circuit 12 compares xe2x80x9csource terminal transmission rate of an original routexe2x80x9d with xe2x80x9camount of idle frequency bandxe2x80x9d calculated on the basis of the amount of use of frequency band in the bypass route to determine whether or not the original path can assure a frequency band therefor on the bypass route. When it is possible to assure the frequency band on the bypass route, the control circuit 12 outputs a self healing switch instruction signal to the SW circuit 15 to switch the original path to a path on the bypass route.
FIG. 2 is an illustration for explaining an example of frequency band assuring. When the amount of idle frequency band of the bypass route, that is, a value of the physical circuit capacity subtracted by the amount of use of frequency band of the bypass route, exceeds the source terminal transmission rate of the original route, it can be determined that the frequency band assuring is possible.
A problem in this case is that, in order to perform the self healing process, it is generally necessary to perform frequency band calculations of the original and the bypass routes and to rewrite a routing table and, therefore, there is a difference produced in time between a time when a frequency band is assured in the bypass route and a time when the switching of the original path to the bypass route is completed. Particularly, in the case of the ABR communication services, the source terminal transmits data at a rate corresponding to the state of network without preliminarily reserving a frequency band therefor. Therefore, when the self healing is to be performed, there is. a possibility that the transmission rate of the source terminal of the original path (failure path) or the transmission rate of the source terminal of the bypass path at the completion of the switching is increased compared with that at the band assuring time of the bypass route, that is, the start time of the self healing execution. FIGS. 3 and 4 show examples of such case.
FIGS. 3 and 4 show cases where the source terminal transmission rates of the original route and the bypass route are increased at the completion of the switching to the bypass route compared with those at the band assuring time of the bypass route, respectively. As shown in FIG. 3, when the source terminal transmission rate of the original route at the switching time is increased compared with that at the band assuring time, a frequency band for the increment of the rate can not be assured necessarily. Therefore, a portion of cell, which corresponds to the increment of transmission rate, exceeds the physical circuit capacity of the bypass route and loss of the cell portion may occur. Further, as shown in FIG. 4, when the source terminal transmission rate of the bypass route at the time of switching is increased compared with that at the band assuring time, a sum of a frequency band therefor and an assured frequency band for the source terminal transmission rate to be switched from the original route may exceed the physical circuit capacity. Therefore, there is a possibility of loss of cell on the bypass route immediately after the switching thereto by the self healing even in such case.
An object of the present invention is to provide a self healing method for continuing a cell transfer between a source terminal and a destination terminal when a failure occurs in an original path by assuring a frequency band on a bypass route and switching the original path to the bypass route without loss of cell and an ATM cell switch device for use in the same method.
According to a first aspect of the present invention, a self healing method for use in the ABR communication service in which, when a failure occurs in a path in which a cell transfer from a source terminal to a destination terminal is being performed at a variable rate, the cell transfer is maintained by assuring a frequency band in a bypass route different from the path in which the failure occurs and switching the path to the route, is featured by comprising the step of fixing upper limits of the cell sending rate at respective source terminals in both the path in which the failure occurs and the bypass route, during a time period from a time when a frequency band is assured in the bypass route to a time of completion of path switching.
In order to fix the upper limit of cell sending rate, an ATM cell switch device which performs the path switching and an ATM cell switch device on the bypass route set contents of ER (Explicit Rate) fields of BRM (Backward Resource Management) cells (referred to as xe2x80x9cER-in-BRM fieldsxe2x80x9d, hereinafter) to be transferred to the respective source terminals to a value of the sending rate of one of the source terminals which is used at the time when the frequency band is assured in the bypass route during the time period from a time when the frequency band is assured in the bypass route to a time of completion of path switching. The respective source terminals set sending rates thereof according to the contents of the ER-in-BRM fields. Further, contents of ER fields of FRM (Forward Resource Management) cells (referred to as xe2x80x9cER-in-FRM cellsxe2x80x9d, hereinafter) to be transferred to the respective destination terminals are preferably set to the same value. The FRM cell is aliased by the destination terminal as a BRM cell. Since there is a possibility of loss of cell due to the failure on the original route, it is preferable to produce and send FRM cell and BRM cell by the ATM cell switch devices in addition to those transferred from the source terminals and the destination terminals.
Separately from the above mentioned matters or in combination with them, it is possible to set a NI (No Increase) bit of the BRM cell (referred to as xe2x80x9cNI-in-BRM bitxe2x80x9d, hereinafter) to a predetermined value in the ATM cell switch device and to prevent the cell sending rate from being increased on the side of the source terminal which receives such BRM cell. In such case, it is preferable to set a NI bit in FRM cell (referred to as xe2x80x9cNI-in-FRM bitxe2x80x9d, hereinafter) similarly.
Further, it is possible, in order to fix the upper limit of the cell sending rate, to utilize an EFCI (Explicit Forward Congestion Indication) bit of a data cell (referred to as xe2x80x9cEFCI-in-Data bitxe2x80x9d, hereinafter). That is, the ATM cell switch devices set the EFCI-in-Data bit to a predetermined value and transfer it to corresponding destination terminal and the respective destination terminals send BRM cells having NI bit and/or EFCI bit set to predetermined value to corresponding source terminals. According to the BRM cell, the source terminal prevent the cell sending rate thereof from being increased by its self According to a second aspect of the present invention, an ATM cell switch device is featured by comprising receiving means for receiving a cell from a source terminal to which the cell is input through a transmission link, switch means for routing the received cell on the basis of the VPI and the VCI of the received cell, transmission means for sending the cell toward a destination terminal, control means for managing a sending rate of the source terminal, instructing a path switching to said switch means during an execution of self healing and judging whether or not a frequency band is assured in a bypass route and means for preventing an increase of source terminal sending rate by sending a control information for preventing the sending rate of the source terminal from being increased during a time period from a time when the control means judges the frequency band assuring in the bypass route to a time when the path switching is completed by the switch means.
The preventing means may comprise a source terminal sending rate measuring circuit for counting cells received from said source terminal, a buffer circuit for holding a source terminal sending rate, that is, cell count rate, (CCRSH) at a time when the frequency band of the bypass route is assured and an ER field change circuit for changing the ER-in-BRM field transferred from the destination terminal to the source terminal for circuit control to the CCRSH. In such case, it is preferable to provide an ER field change circuit for changing the content of the ER in-FRM field transferred from the source terminal side to the destination terminal side to the CCRSH. Further, the preventing means preferably comprises a BRM cell producing/insertion circuit for newly producing a BRM cell having the ER field set to the CCRSH and sending it toward the source terminal and an FRM cell producing/insertion circuit for producing an FRM cell similarly and sending it toward the destination terminal.
Separately from this or in combination therewith, the preventing means preferably comprises a NI-in-BRM bit change circuit for changing a NI bit of BRM cell transferred from the side of the destination terminal toward the side of the source terminal to xe2x80x9c1xe2x80x9d. In this case, too, the preventing means preferably further comprises a NI-in-FRM bit change circuit for changing a NI bit of an FRM cell transferred from the side of the source terminal toward the side of the destination terminal to xe2x80x9c1xe2x80x9d, a BRM cell producing/insertion circuit for newly producing a BRM cell having NI bit set with xe2x80x9c1xe2x80x9d and sending it toward the source terminal and an FRM cell producing/insertion circuit for producing an FRM cell similarly and sending it toward the destination terminal.
Further, the preventing means may comprise an EFCI-in-Data bit change circuit for changing the EFCI bit of the data cell transmitted from the source terminal to the destination terminal to xe2x80x9c1xe2x80x9d.
As described, the preventing circuit sends a control information for preventing the sending rate from being increased to the source terminal directly or through the destination terminal indirectly during the time period in execution of the self healing from the time when the frequency band of the bypass route is assured to the time when the switching of route is completed. Thus, it is possible to prevent the source terminal sending rates of the original route and the bypass route from being increased and hence to execute the self healing of the ABR without loss of cell.